End mills, which are made of high-hardness materials such as cemented carbide and therefore are suitable for cutting of high-hardness materials and high-speed cutting, are increasing. Ball end mills are one kind of such end mills. A cutting edge of a ball end mill made of a high-hardness material is more likely to be chipped than that of a ball end mill made of high-speed steel or the like. Therefore, the chipping resistance of a ball end cutting edge at the tip and the chip removal performance are improved to prolong the tool life or improve the roughness of a machined surface. For example, Patent Literature 1 to 3 disclose such ball end mills.
Patent Literature 1 discloses a ball end mill, in which two ball end cutting edges of a two-flute end mill are arranged over the center core of the end mill, and thereby a chisel edge (chisel blade) having a specified length is provided in the rotation center.
In this ball end mill, the length of the chisel edge is specified to increase the ratio of a flute existing anterior to the chisel edge, thereby improving the chip removal performance.
Patent Literature 2 discloses a ball end mill, in which two ball end cutting edges of a two-flute end mill are arranged over the center core of the end mill, and a chisel edge having a limited chisel angle is provided in the rotation center. In this ball end mill, since each ball end cutting edge is arranged over the center core, the thickness of the edge at the nose section is increased, and thereby the edge in the rotation center is strengthened. Further, since the chisel angle of the chisel edge positioned in the rotation center is limited, chips produced by cutting with the chisel edge are favorably removed.
Also in a ball end mill disclosed in Patent Literature 3, two ball end cutting edges of a two-flute end mill are arranged over the center core of the end mill, and a chisel edge is provided in a rotation center. In this ball end mill, an inner end of each ball end cutting edge is extended to the other ball end cutting edge side beyond the rotation center of the tool, and thus so-called plunging is allowed.
The ball end mills disclosed in Patent Literature 1 to 3 are identical in that the ball end cutting edges are arranged over the center core, the inner end of each ball end cutting edge is extended to the other ball end cutting edge side beyond the rotation center of the tool, and the chisel edge is provided in the rotation center.
Patent Literature 1: Japanese Patent No. 3833875
Patent Literature 2: Japanese Laid-Open Patent Publication No. 2000-233311
Patent Literature 3: Japanese Laid-Open Patent Publication No. 2001-9624